The present invention generally relates to systems for converting a number of scanning lines, and more particularly to a system in which the sampling frequency of a discrete signal sequence related to a plurality of picture elements arranged along a vertical direction with respect to a picture frame is converted into a predetermined frequency, to convert a video signal of a first television system into a video signal of a second television system which uses a number of scanning lines which are different from those used in the first television system.
As is well known, a picture frame of a television receiver and the like is formed as an electron beam scans horizontally from the left to right, for example, with the horizontal scan being performed vertically from the upper part to the lower part of the picture frame. An information regarding the horizontal scanning line can be considered as a time-sequential signal of a discrete information regarding picture elements on the horizontal scanning line. Accordingly, the picture frame can be considered as being formed from a plurality of picture elements arranged in a matrix form. That is, the picture frame can be considered as being constructed from a two-dimensional discrete information obtained by a discrete signal sequence of a time interval (a predetermined sampling period) corresponding to intervals between adjacent picture elements along the horizontal scanning direction.
The television system is not unified throughout the world. With respect to the number of scanning lines in a frame of the video signal, there are television systems (such as the NTSC system) using 525 scanning lines, television systems (such as the PAL system and the SECAM system) using 625 scanning lines, and the like. Hence, as well known, if a digital video signal which is a discrete signal sequence of the above described predetermined sampling frequency is to be reproduced by a television receiver of a different television system, it becomes necessary to perform a television system conversion. For example, there was a conventional apparatus for converting a digital video signal of a television system using 625 scanning lines in a frame, into a digital video signal of a television system using 525 scanning lines in a frame. In this conventional apparatus, decimation was performed to discard 4 scanning lines in every 25 scanning lines, in order to convert the number of scanning lines to 525 from 625.
However, in the above conventional apparatus, in a case where the above described system conversion is performed to convert the number of scanning lines to 525 from 625 when the digital video signal indicates an inclined line in the picture frame, for example, the inclined line in the picture frame became partially discontinuous after the system conversion. Moreover, since the above decimation is performed with respect to the horizontal scanning lines, there was a disadvantage in that degradation was introduced in the vertical resolution.
As another conventional apparatus for converting the number of scanning lines, there was an apparatus which performed the system conversion by considering the video signal as a space transfer function f(x, y) having space frequencies x and y, by using Hadamard's conversion. However, sufficient research has not been made in this field. Further, since this method optimizes approximate values, there were various disadvantages in that the apparatus required a considerably large hardware the time required for the calculation was extremely long, and uncertainty was introduced in the data obtained.
Another method may be considered in which a discrete sequential input related to the picture elements of a first television system is converted into a discrete sequential output related to the picture elements of a second television system. For example, when obtaining a picture element g(x.sub.n1, y.sub.n2), the following equations (1) or (2) stands if the input signal is designated by f(x, y). ##EQU1## In the above equations (1) and (2), T1 indicates the sampling period of the input along a horizontal axis, and T2 indicates the sampling period of the output along the horizontal axis. The sampling points (picture elements) of the output are thus determined by the picture elements of the input, by use of the equation (1) or (2). However, it can be seen from the above equations that the calculation will become extremely complex, and as a result, uncertainties are introduced in the results obtained. That is, because of the complexity involved in calculating the above equations, there is a disadvantage in that the circuit construction will become complex and that errors will inevitably be introduced during the complex calculation.